Capacitors are electrical components that store electrical energy in an electromagnetic field between electrodes that are separated by a dielectric insulator. Each electrode carries a charge that is opposite in polarity to the charge on the other electrode. Capacitors find many applications in a wide variety of electric circuits. Some applications require the capacitor to withstand a high voltage between its electrodes. For example, some camera flash devices produce light by an electric discharge in a gas. A high voltage is required to create the discharge. A power converter transforms a low voltage obtained from a battery into a high voltage, which is stored on the capacitor and used to trigger the flash. In another example, external and implantable defibrillators deliver a high voltage electrical countershock to the heart. The countershock restores the heart's rhythm during cardiac arrhythmias such as life-threatening ventricular fibrillation. In an implantable defibrillator, a power converter transforms a low voltage (e.g., approximately 3.25 Volts), obtained from a battery, into a high voltage (e.g., approximately 750 Volts), which is stored on capacitors and used to defibrillate the heart.
Electrolytic capacitors are used in cameras, defibrillators, and for other electric circuit applications. An electrolytic capacitor includes two electrodes: an anode and a cathode. The dielectric insulator between the anode and cathode is formed by anodizing the anode electrode (i.e., growing an oxide on the anode). The anode and cathode electrodes are physically separated from each other by a porous separator that is soaked with a conductive electrolyte solution. The electrolyte acts as a part of the cathode electrode. A parallel plate capacitor is formed by a substantially parallel planar arrangement of superjacent anode and cathode plates. A separator is interposed in between the anode and cathode electrode plates. A cylindrical capacitor is formed by winding anode, cathode, and separator strips into a spiraled cylindrical roll. For electrically connecting the capacitor in an electric circuit, tabs are joined to the anode and cathode. The tabs protrude outwardly from an end of the cylinder so that the capacitor can be connected in the electric circuit.
By maximizing the energy density of a capacitor, its volume can be reduced. This is particularly important for implantable medical devices, such as implantable defibrillators, since the defibrillation energy storage capacitor occupies a significant portion of the implantable defibrillator device. Smaller implantable defibrillator devices are desired. Smaller defibrillators are easier to implant in a patient. Also, for a particular defibrillator size, a smaller capacitor allows the use of a larger battery, which increases the effective usable life of the implanted device before surgical replacement is required. Thus, one goal of implantable defibrillator design is to maximize capacitor energy density and minimize capacitor volume.
The energy density of a capacitor increases in proportion to a corresponding increase in the surface area of the anode. For example, an anode having a particular macroscopic surface area can be roughened to increase its microscopic surface area. The capacitance per unit of macroscopic surface area, which is sometime referred to as the foil gain of the capacitor, increases as a result of roughening techniques. One such roughening technique includes tunnel etching tiny openings partially or completely through the anode electrode strip.
Anode surface area is further increased by stacking multiple tunnel-etched anodes, thereby obtaining even more surface area and, in turn, an even capacitance per unit area of the anode stack. However, in such multi-anodic capacitors, an electrical connection to each anode in the stack is still required. One approach to making an electrical connection to each anode in the stack is to join a connecting tab to each anode. Individually joining such tabs to each anode, however, increases the volume of the capacitor. Cylindrical capacitors, for example, will bulge as a result of each tab that is inserted into the roll and joined to an anode strip. Not only does this disadvantageously increase the capacitor volume, it increases reliability concerns. Joining tabs to the anode strips causes mechanical stresses, such as at the joints between the tab and the anode strip, and within the anode strip near the edges of the tab. Tunnel-etched anode strips are extremely brittle, making the anodes highly susceptible to such mechanical stresses. Thus, significant disadvantages arise from providing separate tabs to individually contact each anode strip.
Capacitor volume can be reduced slightly by interposing a shared tab in between two adjacent anode plates in the anode stack, such as described in Pless et al. U.S. Pat. No. 5,131,388, entitled, "IMPLANTABLE CARDIAC DEFIBRILLATOR WITH IMPROVED CAPACITORS." This technique still requires at least one tab for every two adjacent anode plates, thereby limiting the reduction in capacitor volume that is obtained. Even more disadvantageously, the Pless et al. patent requires that each double anode is formed by welding two anode plates together with an aluminum strip (i.e., a tab) between them for electrical contact. Not only does such welding add complexity and expense to the manufacture of the capacitor, it causes reliability concerns because the extremely brittle tunnel etched anodes may be further weakened by the welding process. The process of joining anode plates by welding is also described in Elias et al. U.S. Pat. No. 5,660,737 entitled "PROCESS FOR MAKING A CAPACITOR FOIL WITH ENHANCED SURFACE AREA," in which each anode plate must have an electrical connection to the anode terminal, and the anode plates are joined to each other and a tab connection by welding.
Another example of a multi-anodic capacitor is described in MacFarlane et al. U.S. Pat. No. 5,584,890 entitled "METHODS OF MAKING MULTIPLE ANODE CAPACITORS." MacFarlane et al. describes a triple layer anode stack in which an opening in the intermediate anode layer receives an inserted tab that is shared between the adjacent three anodes. This technique still requires at least one tab for every three adjacent anode plates, thereby limiting the reduction in capacitor volume that is obtained. Even more disadvantageously, the MacFarlane et al. patent requires that each triple anode stack is formed by joining the three anode plates together using cold welding, laser welding, or arc welding, even though, as recognized by MacFarlane et al., "highly etched oxidized anode foil is brittle and difficult to join."
Thus, there is a need for further reducing capacitor volume, increasing capacitor reliability, and reducing cost and complexity of the capacitor manufacturing process, for multi-anodic capacitors used in implantable defibrillators, camera photoflashes, and other electric circuit applications.